This invention relates to a process for the preparation of a powder coating composition containing a copolymer of a glycidyl group- or methylglycidyl group-containing unsaturated monomeric compound with an ethylenically unsaturated comonomer.
Powder coating compositions are now used in a wide variety of fields in place of liquid coating compositions which cause, during coating and hardening, air pollution by the generation of organic vapors. In particular, powder coating compositions containing a glycidyl group-containing acrylic resin and a polybasic acid curing agent are advantageously used because they give coatings having a beautiful appearance and excellent weatherability.
One known method for the preparation of such a glycidyl group-containing acrylic resin powder coating composition includes suspension-, bulk- or solution-polymerizing a glycidyl group-containing unsaturated monomeric compound with an ethylenically unsaturated monomer to obtain an acrylic copolymer-containing product. The product is treated for the removal of volatile matters such as a solvent and is subsequently coarsely pulverized and then dry-mixed with a curing agent, a pigment and other additives using a mill. The milled mixture is then melted, kneaded, pelletized, ground and sieved to obtain the powder coating composition.
The known method, however, has the following drawbacks. Namely, when suspension polymerization is adopted in the above method, a large amount of a chain transfer agent must be used in order to obtain a relatively low molecular weight copolymer. When a bulk polymerization is adopted, on the other hand, there is caused a drawback that a high polymerization degree cannot be attained. With a solution polymerization using an inert solvent such as toluene or xylene, a step for removing the solvent from the polymerization product must be performed at a high temperature under a high vacuum for a long period of time.
Additionally, since the above acrylic copolymer obtained by the removal of volatile matters is in the form of a bulk at a low temperature, it is necessary to coarsely pulverize same before mixing with the curing agent, pigment and additives. Further, kneading, pelletizing and grinding of the mixture must be carried out to obtain a powder coating composition.